This invention relates generally to programmable cardiac stimulating devices. More specifically, the present invention is directed to an implantable stimulation device and associated method for controlling the electrode sensing and stimulation configurations and the activation sequence in a multi-chamber cardiac stimulation device using noninvasive programming techniques.
In a normal human heart, the sinus node, generally located near the junction of the superior vena cava and the right atrium, constitutes the primary natural pacemaker initiating rhythmic electrical excitation of the heart chambers. The cardiac impulse arising from the sinus node is transmitted to the two atrial chambers, causing a depolarization known as a P-wave and the resulting atrial chamber contractions. The excitation pulse is further transmitted to and through the ventricles via the atrioventricular (A-V) node and a ventricular conduction system causing a depolarization known as an R-wave and the resulting ventricular chamber contractions.
Disruption of this natural pacemaking and conduction system as a result of aging or disease can be successfully treated by artificial cardiac pacing using implantable cardiac stimulation devices, including pacemakers and implantable defibrillators, which deliver rhythmic electrical pulses or other anti-arrhythmia therapies to the heart at a desired energy and rate. One or more heart chambers may be electrically stimulated depending on the location and severity of the conduction disorder.
Cardiac pacemakers conventionally stimulate a heart chamber by applying current pulses to cardiac tissues via two electrodes, a cathode and an anode. Standard pacing leads are available in either of two configurations, unipolar leads or bipolar leads, depending on the arrangement of the electrodes of a particular lead. A unipolar pacing lead contains a single electrode, normally the cathode, which extends pervenously distal from the pacemaker in an insulating enclosure until it is adjacent to the tip of the lead where the insulation is terminated to provide for electrical contact of the cathode with the heart tissue. The anode provides a return path for the pacing electrical circuit. For a unipolar lead, the anode is the pacemaker case.
A bipolar lead contains two electrodes within an insulating sheath, an anode that extends distal from the pacemaker to a position adjacent to, but spaced from, the electrode tip, and a cathode that also extends distal from the pacemaker, but terminates a short distance distal of the anode, at the lead tip. The anode commonly takes the form of a ring having greater surface area than the cathode tip. An insulating barrier separates the cathode and anode of a bipolar lead. In present-day pacemakers, circuits for pacing and sensing, which determine tip, ring and case electrode connections, are provided. Thus, the pacemakers can be programmed via telemetry for either bipolar or unipolar operation with respect to either sensing or pacing operations.
A single-chamber pacemaker delivers pacing pulses to one chamber of the heart, either one atrium or one ventricle, via either a unipolar or bipolar electrode. Single-chamber pacemakers can operate in either a triggered mode or a demand mode. In a triggered mode, a stimulation pulse is delivered to the desired heart chamber at the end of a defined time-out interval to cause depolarization of the heart tissue (myocardium) and it""s contraction. The stimulating pulse must be of sufficient energy to cause depolarization of the heart chamber, a condition known as xe2x80x9ccapture.xe2x80x9d The lowest pulse energy required to achieve capture is termed xe2x80x9cthreshold.xe2x80x9d The pacemaker also delivers a stimulation pulse in response to a sensed event arising from that chamber when operating in a triggered mode.
When operating in a demand mode, sensing and detection circuitry allow for the pacemaker to detect if an intrinsic cardiac depolarization, either an R-wave or a P-wave, has occurred within the defined time-out interval. If an intrinsic depolarization is not detected, a pacing pulse is delivered at the end of the time-out interval. However, if an intrinsic depolarization is detected, the pacing pulse output is inhibited to allow the natural heart rhythm to preside. The difference between a triggered and demand mode of operation is the response of the pacemaker to a detected native event.
Dual chamber pacemakers are now commonly available and can provide either trigger or demand type pacing in both an atrial chamber and a ventricular chamber, typically the right atrium and the right ventricle. Both unipolar or bipolar dual chamber pacemakers exist in which a unipolar or bipolar lead extends from an atrial channel of the dual chamber device to the desired atrium (e.g. the right atrium), and a separate unipolar or bipolar lead extends from a ventricular channel to the corresponding ventricle (e.g. the right ventricle). In dual chamber, demand-type pacemakers, commonly referred to as DDD pacemakers, each atrial and ventricular channel includes a sense amplifier to detect cardiac activity in the respective chamber and an output circuit for delivering stimulation pulses to the respective chamber.
If an intrinsic atrial depolarization signal (a P-wave) is not detected by the atrial channel, a stimulating pulse will be delivered to depolarize the atrium and cause contraction. Following either a detected P-wave or an atrial pacing pulse, the ventricular channel attempts to detect a depolarization signal in the ventricle, known as an R-wave. If no R-wave is detected within a defined atrial-ventricular interval (AV interval or delay), a stimulation pulse is delivered to the ventricle to cause ventricular contraction. In this way, rhythmic dual chamber pacing is achieved by coordinating the delivery of ventricular output in response to a sensed or paced atrial event. Mounting clinical evidence supports the evolution of more complex cardiac stimulating devices capable of stimulating three or even all four heart chambers to stabilize arrhythmias or to re-synchronize heart chamber contractions (Ref: Cazeau S. et al., xe2x80x9cFour chamber pacing in dilated cardiomyopathy,xe2x80x9d Pacing Clin. Electrophsyiol 1994 17(11 Pt 2):1974-9). Stimulation of multiple sites within a heart chamber has also been found effective in controlling arrhythmogenic depolarizations (Ref: Ramdat-Misier A., et al., xe2x80x9cMultisite or alternate site pacing for the prevention of atrial fibrillation,xe2x80x9d Am. J. Cardiol., 1999 11;83(5b):237D-240D).
In order to achieve multi-chamber or multi-site stimulation in a clinical setting, conventional dual-chamber pacemakers have now been used in conjunction with adapters that couple together two leads going to different pacing sites or heart chambers. Reference is made to U.S. Pat. No. 5,514,161 to Limousin in which a triple chamber cardiac pacemaker, with the right and left atrial combined with a right ventricular lead, is described. Cazeau et al. (Pacing Clin. Electrophsyiol 1994 17(11 Pt 2):1974-9) describe a four chamber pacing system in which unipolar right and left atrial leads are connected via a bifurcated bipolar adapter to the atrial port of a bipolar dual chamber pacemaker. Likewise, unipolar right and left ventricular leads are connected via a bifurcated bipolar adapter to the ventricular channel. The left chamber leads were connected to the anode terminals and the right chamber leads were connected to the cathode terminals of the dual chamber device. In this way, simultaneous bi-atrial or simultaneous bi-ventricular pacing is achieved via bipolar stimulation but with several limitations.
Firstly, this configuration of bipolar stimulation is distinctly different from a conventional bipolar lead configuration wherein both the cathode and anode are located a short distance apart, approximately one centimeter, on the same lead. In the bi-chamber pacing configuration described above, the anode and cathode are in fact located on two different leads positioned in two different locations, several centimeters apart. In addition, since the tip electrode of one lead is forced to be the anode, and this has a significantly smaller surface area than the anode of a classic bipolar lead, the relative resistance or impedance is higher with this lead system. In such a bipolar, bi-chamber pacing configuration, the threshold energy is likely to be relatively higher than in conventional bipolar stimulation in part because of the higher impedance of the electrode system. In addition, the electrode used for stimulation in the left heart chamber is usually within the coronary sinus or a cardiac vein, not making direct contact with the myocardium. As such, the energy needed to accommodate bi-chamber stimulation will usually be higher than that which is commonly required for single chamber stimulation using bipolar leads.
A potential risk that exists when higher output settings are used, as may be needed to ensure bi-chamber stimulation, is cross-chamber capture, also known as cross-stimulation (Ref: Levine P A, et al., Cross-stimulation: the unexpected stimulation of the unpaced chamber, PACE 1985: 8: 600-606). If bi-atrial stimulation is delivered in a bipolar configuration across one electrode located in the right atrium and another electrode located in the left atrium, which in actuality is the coronary sinus which lies between the left atrium and left ventricle, the stimulation energy could conceivably be high enough to inadvertently capture one or both ventricles simultaneously. Such cross-chamber capture is a highly undesirable situation in that the upper and lower chambers would contract against each other causing severe cardiac output perturbation. This is also likely to occur with bipolar bi-ventricular stimulation with respect to cross-stimulation of the atrial chambers if the left ventricular lead located within a cardiac vein is in close anatomic proximity to the left atrium and high outputs are required to assure capture.
Another limitation of the multi-chamber stimulation systems described above is that simultaneous stimulation of left and right chambers, as required when two leads are coupled together by one adapter, is not always necessary nor desirable. For example, in some patients conduction between the two atria may be compromised, however the pacemaking function of the sinus node in the right atrium may still be normal. Hence, detection of an intrinsic depolarization in the right atrium could be used to trigger delivery of a pacing pulse in the left atrium. Since an intrinsic depolarization has occurred in one chamber, simultaneous stimulation of both chambers in this situation is unnecessary.
In another example, when inter-atrial or inter-ventricular conduction is intact, stimulation in one chamber may be conducted naturally to depolarize the second chamber. A stimulation pulse delivered in one chamber, using the minimum energy required to depolarize that chamber, will be conducted to the opposing chamber thus depolarizing both chambers. In this case, stimulation of both chambers simultaneously would be wasteful of battery energy.
Another limitation is that, in the presence of an inter-atrial or inter-ventricular conduction defect, one may want to control the interval between a sensed or paced event in one chamber and delivery of a stimulation pulse to the other chamber. If pacing is required in both chambers, the control of the sequence of the stimulation pulse delivery to each chamber, rather than the simultaneous delivery of stimulation pulses, may be desirable in order to achieve a specific activation sequence that has hemodynamic benefit.
Yet another limitation is that, once implanted, the designation of cathode and anode assignments is fixed and cannot be reassigned in order to determine the polarity that results in the lowest stimulation thresholds, to achieve a desired directionality of the stimulation delivery or to obtain the optimal sequencing of stimulation and/or sensing to optimize hemodynamic function. Typically, the electrode in the right chamber is connected to the cathode terminal and the electrode in the left chamber is connected to the anode terminal. In other cases, the electrode in the left chamber is connected to the cathode terminal while the right chamber electrode is connected to the anode. In some patients, a lower stimulation threshold or an improved excitation pattern or perhaps even hemodynamic benefit might be achieved by reversing the cathode and anode locations yet this cannot be done without operative intervention.
In the first generation of multi-chamber devices, an adapter was required to connect multiple leads to a conventional dual chamber device, a requirement that adds cost and time to the implant procedure. Adapters can be cumbersome and an additional site for potential lead breakage or discontinuity, essentially adding bulk and a xe2x80x9cweak linkxe2x80x9d to the implanted system. In certain current devices, adapters are no longer required. The connection between leads is hardwired internally in the connector block coupling the ventricular leads to the ventricular channel and the atrial leads to the atrial channel. While this design advantageously eliminates the need for adapters, the hardwire connections preclude the potential to non-invasively adjust the polarity orientation. This also prevents introducing separate timing between stimulation pulses delivered to each chamber or responding with any programmable delays to a sensed event by delivery of an output pulse to the other chamber.
To address some of these limitations, Verboven-Nelissen proposes a method and apparatus that includes a multiple-chamber electrode arrangement having at least two electrodes placed to sense and/or pace different chambers or areas of the heart. Reference is made to U.S. Pat. No. 5,720,518. The proposed method involves switching from a bipolar to a unipolar configuration during sensing for determining the origination site of a detected depolarization signal. If the signal is determined to have arisen from the SA node in the right atrium, a conduction interval is applied to allow the cardiac signal to properly propagate to the other heart chambers. If no cardiac signal is detected in another cardiac chamber, for example, the left atrium, then pacing is initiated in that chamber at the end of the conduction interval. In this example, the interval is equal to the inter-atrial conduction time (i.e. the time required for a P-wave cardiac signal to propagate from right atrium to left atrium). However the inter-atrial conduction time may vary over time and the time for an excitation pulse to propagate from the right chamber to the left chamber may be different than the propagation time from the left chamber to the right chamber. In addition, the conduction time from the right atrium to the left atrium may vary from that required to go from the left atrium to the right atrium. Depending on the site of origin of the detected depolarization, it may be hemodynamically beneficial to control the coupling interval between the detected depolarization and the triggered output to the other chamber. U.S. Pat. No. 5,720,518 does not address the ability to control the interval between detection and stimulation within the atria or ventricles in the setting of multisite stimulation.
Reference is also made to U.S. Pat. No. 5,902,324 to Thompson et al. in which a multi-channel pacing system having two, three or four pacing channels, each including a sense amplifier and pace output pulse generator, is described. A pacing pulse or detection of a spontaneous depolarization in one of the right or left heart chambers is followed by a short conduction delay window. A pacing pulse that would otherwise be delivered at the termination of the conduction delay window in the opposing heart chamber is inhibited if the conducted depolarization wave is sensed within the conduction delay window. While the duration of the conduction delay window can be programmed, no method is provided by which to select the optimal interval between chamber depolarizations.
Patients with marked hemodynamic abnormalities may benefit from multi-site or multi-chamber pacing that controls the activation sequence of the heart chambers. Precise control of the activation sequence may improve the coordination of heart chamber contractions resulting in more effective filling and ejection of blood from the heart. Patients with hemodynamic abnormalities often have conduction defects due to dilation of the heart or other causes. Yet, even in patients with intact conduction, precise control of the timing and synchronicity of heart chamber contractions may provide hemodynamic benefit.
There remains an unmet need, therefore, for a multi-chamber or multisite cardiac stimulation device that allows independent stimulation and sensing at multiple sites within the heart as well as flexible selection of stimulation sequence and timing intervals between these stimulation sites. It would thus be desirable to provide a multisite or multichamber cardiac stimulation device having independent sensing and output circuitry for each pacing site. It would further be desirable to allow flexible selection of sensing and stimulation polarity for each stimulation site, including the designation of cathode and anode assignment during bi-chamber stimulation. Further, it would be desirable to provide flexible programming of the stimulation sequence and timing intervals associated with multisite or multichamber pacing. Different timing intervals should be advantageously selectable depending on the origination site of a detected depolarization wave or a desired directionality of depolarization in order to achieve optimal hemodynamic or electrophysiological benefit for the patient.
The present invention addresses this need by providing an implantable multichamber or multisite cardiac stimulation device in which the electrode configurations for sensing and stimulation are flexibly programmable, and the stimulation sequence between multiple sites can be precisely controlled.
One aspect of the present invention is to provide a plurality of connection ports, preferably two through four connection ports, that allow independent connection to the stimulation device of each electrical lead associated with a particular stimulation site in the heart. Each connection port further provides a unique terminal for making electrical contact with only one electrode such that no two electrodes are required to be electrically coupled. Furthermore, each electrode, whether residing on a unipolar, bipolar or multipolar lead, may be selectively connected or disconnected through programmable switching circuitry that determines the electrode configurations to be used for sensing and for stimulating at each stimulation site.
Another aspect of the present invention is a unique sensing circuit associated with each stimulation site such that depolarizations occurring at each stimulation site can be detected independently of events occurring at other sites within the heart. This independent sensing advantageously allows the location of a detected depolarization to be recognized by the stimulation device. The desired electrodes to be used for sensing in a specific heart chamber or at a specific site within a heart chamber are connected to the input of the sensing circuit via programmable switching circuitry.
Still another aspect of the present invention is a unique output circuit associated with each stimulation site such that each site can be stimulated independently of other sites or on a precisely timed basis triggered by events occurring at other sites. The electrodes used for stimulation at a specific site may be different than those used for sensing at the same site.
Yet another aspect of the present invention is the ability to program unique coupling intervals for precisely controlling the activation sequence of stimulated sites. Coupling intervals may be defined in relation to the originating location of a detected depolarization or in relation to stimulus delivery at another location. Coupling intervals are advantageously selected in a way that provides optimal hemodynamic benefit to the patient by overcoming various conduction disorders or improving coordination of heart chambers in patients suffering from heart failure. One embodiment of the present invention includes a method for automatically determining the optimal coupling intervals and adjusting the programmed settings based on measurements related to the hemodynamic state of the heart.